The manufacture of a ceramic matrix composite (CMC) part typically includes laying up pre-impregnated composite fibers having a matrix material already present (prepreg) to form the geometry of the part (pre-form), autoclaving and burning out the pre-form, infiltrating the burned-out pre-form with the melting matrix material, and any machining or further treatments of the pre-form. Infiltrating the pre-form may include depositing the ceramic matrix out of a gas mixture, pyrolyzing a pre-ceramic polymer, chemically reacting elements, sintering, generally in the temperature range of 925 to 1650° C. (1700 to 3000° F.), or electrophoretically depositing a ceramic powder. With respect to turbine airfoils, the CMC may be located over a metal spar to form an outer covering over the metal spar or to form only the outer surface of the airfoil.
Examples of CMC materials include, but are not limited to, carbon-fiber-reinforced carbon (C/C), carbon-fiber-reinforced silicon carbide (C/SiC), silicon-carbide-fiber-reinforced silicon carbide (SiC/SiC), alumina-fiber-reinforced alumina (Al2O3/Al1O3), or combinations thereof. The CMC may have increased elongation, fracture toughness, thermal shock, dynamic load capability, and anisotropic properties as compared to a monolithic ceramic structure.
Certain conventional CMC blade architectures include an airfoil, a narrowed neck region, and a dovetail to couple the CMC blade to a rotor. Inserts, located between portions of CMC plies at various locations in the airfoil and dovetail, aid in providing an airfoil and a dovetail that are wider than the narrowed neck region between the airfoil and the dovetail. Under certain operational conditions, this architecture may lead to peel or interlaminar tensile (ILT) stresses, which may result in initiation of a crack at or near the mid-plane within the narrowed neck region of the CMC blade. The resulting interlaminar crack tends to propagate radially along the mid-plane of the CMC blade, and the crack may continue to propagate radially past the narrowed neck region along the ply direction or radial direction, ultimately splitting the CMC blade.